Fluid film journal bearings have long been used to dampen the vibration created by turbomachines. Rotors in aircraft gas turbine engines and industrial centrifugal compressors often use squeeze film damper bearings supported by spring bars to reduce the amount of vibration transmitted from the rotor to the supporting structure. In a fluid film bearing, a thin fluid film forms a buffer between the rotating journal surface and the stationary bearing surface, and dampens vibration from the rotor. In a squeeze film damper bearing, a thin film of fluid is squeezed by two non-rotating cylindrical surfaces. One surface is stationary while the other is positioned by a spring bar support structure and oscillates with the motion of the rotor. The squeezing of the fluid film dampens rotor vibration through the bearing support.
Damping the vibration in a turbomachine provides quiet and comfortable operation of the machine, reduced fatigue stress on the machine and its supports, and a safeguard to the damage that can be caused by unstable vibration. Vibration in a turbomachine is usually caused by a rotating mass imbalance, e.g., rotor, or by aerodynamic forces within the turbine and/or compressor. These vibrations are not static, but vary with the operating speed and operating characteristics of the turbomachine. Turbomachine vibration has a dynamic range that varies in magnitude and frequency with the operating speed of the turbomachine. An optimal bearing must have dynamic damping characteristics tailored to the dynamic range of the vibration being applied to the bearing by the turbomachine.
Since two or more bearings are required to support rotating shafts, the bearings must accommodate slight misalignments between the bearing and the rotor. Misalignments can be either angular misalignment, where the axis of rotation of the journal differs from the axial centerline of the bearing, or bearing elevation misalignments on rotors with three or more bearings. It is exceedingly difficult for each of the bearings supporting the turbomachine to be exactly aligned with the rotor. It is especially difficult to align the three or more bearings that support rigidly coupled rotors for combined cycle gas and steam turbines.
Prior art bearings do not accommodate very well the inevitable elevation misalignments between bearings. These prior art bearings become unevenly loaded when there is misalignment with the rotor or other bearings. Misalignments can cause some bearings to bear an excessive load, while other bearings are lightly loaded. An excessive load on a fluid film bearing can reduce the thickness of the oil film to such an extent that the film is inadequate to prevent metal-to-metal contact between the bearing and the journal surface of the rotor. An inadequate oil film can cause exaggerated metal temperatures, extraordinary metal wear and premature failure of the bearing. In addition, the lightly loaded bearings may vibrate with bearing oil whirl which contributes to, rather than dampens, the vibration transmitted from the turbomachine to the bearing support. Accordingly, the inability of prior art bearings to accommodate misalignments is a serious disadvantage of these bearings.
While prior art fluid film bearings are used to dampen vibration, the rigid supports for these bearings limit the dynamic range of vibration that can be attenuated. These prior art bearings are stiff and have a limited ability to dampen vibration. Squeeze film dampers have been applied to fluid film bearings to increase, albeit slightly, the dynamic range of these bearings. To reduce bearing stiffness, spring bar supports have been arranged circumferentially within these bearings to provide additional flex to the bearing. However, prior art spring bars are bulky because of the manner that they are bolted or otherwise attached to the bearing. These bulky spring bars increase the size of the bearing and are typically much larger than a standard fluid film bearing. It has been exceedingly difficult to replace old bearings with bearings having spring bars because of the additional space required for the new bearings.
The current invention is an improved fluid film journal bearing having a squeeze film damper. This bearing has the advantages of prior fluid film bearings with squeeze dampers along with several additional advantages. These additional advantages include a substantial reduction in the transmission of vibration from the rotor to the structure supporting the turbomachine and a compact size.
The low stiffness of the spring bars provides the present invention with a greater range of stiffness and damping characteristics than is available in prior fluid film bearings. In addition, the lower stiffness of the present bearing allows it to adjust to minor height misalignments between bearings and to a skewed rotor shaft. This ability to accommodate height misalignments assists in maintaining uniform loads on multiple bearings and ensures that an adequate hydrodynamic oil film exists within the bearings. Similarly, the bearings are not susceptible to bearing oil whirl instability resulting from lightly loaded bearings.
The present invention provides for a compact journal bearing having spring bars that fit relatively easily within the space required by existing bearings. The circular array of spring bars in the current invention require no external attachments, e.g., bolts, to the bearing support structure. This is in contrast to prior art spring bars that require cumbersome bolts and other attachments to support the damper within the damper. The size of the inventive bearing is comparable to that of existing rigid fluid film bearings. Accordingly, the present bearing is easily adapted as a retrofit bearing for existing bearings, such as rigid fluid film bearings.
In addition, the spring bars of the present invention can be arranged as a circular array encircling the top and lower halves of the rotor. This arrangement provides a symmetrical stiffness characteristic to the bearing. In the alternative, the spring bars can be limited to the lower half of the bearing to provide greater vertical stiffness than horizontal stiffness. This asymmetrical distribution of stiffness reduces the likelihood of aerodynamic whirl instability in the turbomachine. Accordingly, by properly varying the distribution of the spring bars within the bearing the dynamic vibration characteristics of the bearing can be optimized.
The squeeze film dampers in the current invention include plenum chambers filled with oil positioned adjacent the squeeze film region of the bearing. These plenum chambers supply oil to the squeeze film and thin film dampers and ensure that air is not drawn into the oil of the film. The damping capacity of the oil is reduced if air seeps into the oil. Prior art squeeze film dampers allow air to seep through end seals and into the squeeze film. In the present invention, oil filled plenum chambers between the end seals and the squeeze film prevent air seepage. Accordingly, the end seals can be applied to stop oil leakage from the squeeze film and to increase the damping ability of the squeeze film. But, air cannot seep through the seals into the squeeze film. This more effective squeeze film damper further increases the ability of the current invention to dampen vibration from a turbomachine.